Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Trade Accounts Receivable [Abstract]
Billed		$ 106.5		$ 105.3
Unbilled		19.1		16.9
Less: Allowance for Doubtful Accounts		3.1		1.0
Total Accounts Receivable		122.5		121.2
Inventories – Net [Abstract]
Fuel	[1]	43.9		58.1
Materials and Supplies		48.7		49.1
Raw Materials		2.9		2.7
Work in Progress		1.0		0.0
Finished Goods		8.6		7.5
Reserve for Obsolescence		(0.9)		(0.3)
Total Inventories		104.2		117.1
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause		18.6		10.6
Restricted Cash	[2]	2.2		5.6
Other		19.5		19.5
Total Prepayments and Other Current Assets		40.3		35.7
Property, Plant and Equipment [Line Items]
Net Book Value		3,741.2		3,669.1
Impairment of Long-Lived Assets [Abstract]
Impairment of Real Estate	[3]	0.0		36.3	$ 0.0
Goodwill [Line Items]
Impairment of Goodwill	[4]	3.3	[5]	0.0	0.0
Indefinite-Lived Intangible Assets [Abstract]
Impairment of Intangible Assets		0.0		0.0
Other Non-Current Assets [Abstract]
Contract Payment	[6]	29.6		0.0
Financing Receivable	[7]	11.5		0.0
Restricted Cash	[8]	8.6		8.1
Other		56.8		60.0
Total Other Non-Current Assets		106.5		68.1
Contract Payment Made to Cliffs		31.0
Real Estate Sale Consideration [Abstract]
Total Consideration for Land Inventory		$ 21.0
Down Payment of ALLETE Common Stock (Shares)		0.1
Down Payment of ALLETE Common Stock		$ 8.0
Finance Receivable Term (Years)		5 years
Other Current Liabilities [Abstract]
Customer Deposits		$ 5.4		15.1
Power Sales Agreements		24.6		23.3
Other		43.7		47.7
Total Other Current Liabilities		73.7		86.1
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		136.6		131.4
Power Sales Agreements		113.8		138.1
Contingent Consideration	[9]	25.0		36.6
Other		47.3		42.9
Total Other Non-Current Liabilities		322.7		349.0
Business Segments [Line Items]
Amortization of Power Sales Agreements		22.3		23.2	12.7
Operating Expenses – Other [Abstract]
Impairment of Real Estate	[3]	0.0		36.3	0.0
Impairment of Goodwill	[4]	3.3	[5]	0.0	0.0
Change in Fair Value of Contingent Consideration	[10]	(13.6)		0.0	0.0
Total Operating Expenses – Other		$ (10.3)		36.3	0.0
Income Taxes [Abstract]
More-Likely-Than-Not Percentage		50.00%
Regulated Operations [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 3,095.1		3,114.1
Business Segments [Line Items]
Alternative Revenue Program, Required Collection Period (Months)		24 months
ALLETE Clean Energy [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 532.3		447.3
Goodwill [Line Items]
Impairment of Goodwill		3.3	[5]	0.0	0.0
Business Segments [Line Items]
Amortization of Power Sales Agreements		22.3		23.2	12.7
Operating Expenses – Other [Abstract]
Impairment of Goodwill		3.3	[5]	0.0	0.0
U.S. Water Services [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		12.6		12.2
Goodwill [Line Items]
Impairment of Goodwill		$ 0.0	[5]	0.0
Discount Rate		10.75%
Terminal Growth Rate		5.00%
Reporting Unit Fair Value in Excess of Carrying Value		10.00%
Operating Expenses – Other [Abstract]
Impairment of Goodwill		$ 0.0	[5]	0.0
Corporate and Other [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 101.2		95.5
Business Segments [Line Items]
Minimum Cash Collections Requirement for Real Estate Sales (Percent)		20.00%
Minimum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		2 years
Minimum [Member] | ALLETE Clean Energy [Member]
Goodwill [Line Items]
Discount Rate		8.25%
Minimum [Member] | U.S. Water Services [Member]
Goodwill [Line Items]
Annual Revenue Growth Rate		8.00%
Maximum [Member]
Definite-Lived Intangible Assets [Line Items]
Useful Life (Years)		21 years
Maximum [Member] | ALLETE Clean Energy [Member]
Goodwill [Line Items]
Discount Rate		9.25%
Maximum [Member] | U.S. Water Services [Member]
Goodwill [Line Items]
Annual Revenue Growth Rate		11.00%
Wind Turbine Generators [Member] | ALLETE Clean Energy [Member]
Impairment of Long-Lived Assets [Abstract]
Indicators of Impairment				$ 0.0	$ 0.0
Taconite Harbor [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 90.0
Boswell Units 1 and 2 [Member]
Property, Plant and Equipment [Line Items]
Net Book Value		$ 30.0

[1] Fuel consists primarily of coal inventory at Minnesota Power.

[2] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and collateral deposits required for U.S. Water Services’ standby letters of credit.

[3] See Impairment of Long-Lived Assets.

[4] See Goodwill and Intangible Assets.

[5] The facts and circumstances that led to an impairment of ALLETE Clean Energy’s goodwill primarily relate to lower estimated energy prices in periods not under PSAs. Impairment Charge is included in Operating Expenses – Other on the Consolidated Statement of Income. (See Note 1. Operations and Significant Accounting Policies.) ALLETE Clean Energy’s goodwill was primarily related to the acquisition of Storm Lake II in January 2014.

[6] Contract Payment includes a $31.0 million payment made to Cliffs as part of a long-term PSA between Minnesota Power and Silver Bay Power. The contract payment is being amortized over the term of the PSA. (See Note 11. Commitments, Guarantees and Contingencies.)

[7] On September 22, 2016, ALLETE Properties sold its Ormond Crossings project and Lake Swamp wetland mitigation bank for consideration of approximately $21 million. The consideration included a down payment in the form of 0.1 million shares of ALLETE common stock with a value of $8.0 million. The remaining purchase price will be paid under the terms of a finance receivable due over a five-year period which bears interest at market rates and is collateralized by the property sold.

[8] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and PSAs, and deposits from SWL&P customers in aid of future capital expenditures.

[9] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 6. Acquisitions and Note 9. Fair Value.)

[10] See Note 9. Fair Value.